1. Field of the Invention
The invention relates to a method for use in liquid scintillation counting for detecting, recording and analyzing scintillation phenomena due to ionizing radiation in a device comprising a measuring chamber into which the sample is placed and is detected by means of at least two light detectors.
Liquid scintillation counting is a generally known method for detecting radioactive radiation. The sample is placed in a bottle and the scintillation liquid is added thereto transforming the radiation of the sample into light pulses detected by the photomultiplier tube. There are two photomultipliers operating simultaneously and their combined signal is analyzed. The simultaneous technique eliminates the background caused by thermal noise of the photomultipliers by accepting only the signals detected simultaneously by both tubes.
The drawback of the above technique is the fact that, especially in the measurement of low active isotopes, it is difficult to identify and eliminate the background radiation components of liquid scintillation counting. Nowadays the most advanced devices like, e.g., the 1220 Quantulus.TM. manufactured by Wallac Oy are provided, around the actual measuring chamber, with a guard detector which removes a major part of the occurrences of the sample arising from external sources by means of a non-simultaneous technique.
Furthermore liquid scintillation counters like the 1220 Quantulus.TM. and 1219 SM comprise a pulse shape analyzer which enables separation of the pulses produced by alpha and beta particles in the sample. An optimum separation requires, however, that right setting values are provided at the start of the measurement. Otherwise, the measurement will not succeed. The required settings are affected by e.g. the optical and/or the chemical state of the sample as well as scintillation liquid used.
Also, the photomultiplier tubes cause interference. For reducing background pulses from photomultiplier tubes (cross-talk), the above liquid scintillation counter is provided with an amplitude comparator accepting as real pulses from a sample only those pulses which come from different tubes and which are of equal size within a selected accuracy. The optimum settings of the amplitude comparator depends on the state of the sample and the scintillation liquid used.
The above mentioned liquid scintillation counters can be used to produce four spectra at the most from the sample by means of a multi-channel analyzer, which can be set in a versatile way to separate phenomena appearing in the sample according to the nature and origin of the radiation. Finding the optimal settings for the pulse shape analyzer and the amplitude comparator requires, however, several test measurements to be performed beforehand.